This paper extends Alan John Simmons’s conceptual distinction between Lockean (or consent) and Kantian (or justificatory) conceptions of legitimacy that he applied to the question of the legitimacy of states, to the issue of legitimacy of public decisions. I criticise the consent conception of legitimacy defended by Simmons, and I defend the Rawlsian version of the justificatory conception of legitimacy from his objection. The approach of this paper is distinctive because the two conceptions are assessed by investigating, using the method of reflective equilibrium, their respective prescriptions concerning the treatment of antisocial personality disorder (ASPD) and epidemiologic measures. I argue that the method of reflective equilibrium does not support the consent conception. Considering the issues of treatment of APD and of epidemiologic measures, I argue that the consent conception of legitimacy is not well-equipped for the evaluation of norms that are not strictly self-regarding. This causes a deficit of prescriptions for relevant social responses. Further, by considering the case of responses to epidemics, I argue that such a conception can avoid harmful consequences only by recurring to additional, and independent, premises. This does not cause incoherence but reduces the coherence of a normative system. Finally, the consent conception is not equipped to support social cooperation in an optimal way, which has proved to be necessary in critical conditions, like a pandemic. On the other hand, I argue that the method of reflective equilibrium supports the Rawlsian version of justificatory conception of legitimacy, because of its advantages in handling the indicated issues. In addition, I maintain that this justificatory conception is respectful of freedom and equality of agents as moral self- legislators, and, thus, it is not vulnerable to Simmons’s main criticism.